An electrical current through a first node may be amplified by a bipolar transistor, such that an amplified current passes through a second node. To achieve such an amplified current through the second node based upon current through the first node, the first node may be coupled to a base of a properly biased bipolar transistor, and the second node may be coupled to an emitter of the transistor. In this case, if current decreases through the first node, then current through the second node preferably decreases in a proportionate manner when the bipolar transistor is operating in its linear range.
Nevertheless, higher temperatures typically increase junction leakages between a transistor's collector and its base, thereby negatively impacting external attempts to control the transistor by adjusting current at the transistor's base. Thus, junction leakage currents between a transistor's collector and base may result in current through the second node which fails to decrease in a manner proportionate to current through the first node. This problem is addressed in part by providing a turn-off current path between the first and second nodes. Consequently, as current decreases through the first node, the turn-off current path assists in decreasing a voltage potential between the first and second nodes, until the decreased voltage potential is no longer sufficient to enable current conduction through the transistor. When current conduction through the transistor is no longer enabled, potential negative consequences of junction leakages between the transistor's collector and base are substantially avoided.
Moreover, by assisting the decrease in voltage potential between the first and second nodes, the turn-off current path also facilitates voltage swings between the first and second nodes. By facilitating such voltage swings, a voltage polarity may be inverted more quickly between the first and second nodes, such that a direction of current flow between the first and second nodes may be switched more quickly.
Previous approaches have implemented such a turn-off current path by coupling a resistor between the first and second nodes. A shortcoming of this approach is that the turn-off current between the first and second nodes significantly decreases as the voltage potential decreases between the first and second nodes, which may undesirably extend the time during which current conduction through the transistor is enabled, the time during which potential negative consequences of junction leakages may occur, the time necessary for a voltage swing to occur between the first and second nodes, and the time necessary to switch a direction of current flow between the first and second nodes. Consequently, it is desirable to provide a substantially constant turn-off current in order to quickly disable current conduction through the transistor when current through the first node falls below the level of constant turn-off current.
Thus, a need has arisen for a method and apparatus for bidirectional current conduction between first and second nodes of an electronic circuit, such that a substantially constant turn-off current is provided in either direction between the first and second nodes, such that turn-off current between the first and second nodes does not significantly decrease as the voltage potential decreases between the first and second nodes, and such that undesired extensions are substantially avoided in the time during which current conduction through the transistor is enabled, the time during which potential negative consequences of junction leakages may occur, the time necessary for a voltage swing to occur between the first and second nodes, and the time necessary to switch a direction of current flow between the first and second nodes.